Today the polar bear is the largest carnivorous land mammal, but bones and teeth found in a drawer at the National Museums of Kenya come from a creature that was probably even larger, an early example of giant mammalian predators.

Lack of funds means museums worldwide are loaded with exceptional specimens that have never been properly studied and the National Museums of Kenya have more than most. During a lunch break while working there, Dr Matthew Borths, then at Ohio University, decided to open some neglected-looking drawers. He found a set of bones from what was clearly an enormous carnivore, which had been disregarded because the people who found it at Meswa Bridge, western Kenya, were focused on seeking extinct apes.

The beast, which Borths has named Simbakubwa kutokaafrika, stalked the grasslands of Africa some 22 million years ago. The name comes from the Swahili word for “big lion”, rather than the Disney character, but this was no member of the cat family. Instead, Simbakubwa was a hyaenodont, which, to keep up the confusion, were unrelated to hyaenas, but had similar-looking teeth. The specimen's jaw and terrifying teeth are the most complete we have of its branch of the wider hyaenodont family.

Another palaeontologist might not have known what to make of the bones, but Borths has a background in the study of hyaenodonts and realized he was onto something big.

Three views of the Simbakubwa kutokaafrika jawbones we have, with the equivalent lion jawbone for comparison. Borths & Stevens/Journal of Vertebrate Paleontology

After the death of the non-avian dinosaurs, hyaenodonts were quick off the mark to fill the available niche. Africa’s dominant predator family for 45 million years, with an additional presence on other continents, they diversified into hundreds of species of many different sizes. Meanwhile, cats and dogs and actual hyenas were evolving in Eurasia. When the continents came into contact, the hyaenodonts went into sharp decline. Simbakubwa was among the last, as well as one of the largest, hyaenodonts, although a few species hung on for millions more years.

Together with Professor Nancy Stevens, Borths has published the first description of Simbakubwa in the Journal of Vertebrate Paleontology. Based on its rhino-sized head it could have weighed anything from 280 to 1,500 kilograms (600-3,300 pounds), ranging from the size of a large lion to bigger than a polar bear. It was probably its era's largest African predator and had canine teeth suited to shearing flesh, although those of the specimen are unworn, suggesting it was young. Its molars were suited to cracking bones.

Despite being late among its genus, Simbakubwa was a foreshadowing of what was to come, the oldest really large mammalian carnivore we have found in the fossil record. Changes in the African landscape had allowed herbivores to grow bigger, supporting larger carnivores in turn and this persisted for 10-15 million years.

An estimation of Simbakubwa kutokaafrika's size next to a human who appears insufficiently terrified. Mauricio Anton